Help: Fix JOIN generator expression syntax example

Fix syntax of the JOIN generator expression example,
from `$<JOIN,list,glue>` to `$<JOIN:list,glue>`.

Help/manual/cmake-generator-expressions.7.rst

@@ -571,7 +571,7 @@ List Transformations
   :ref:`$\<LIST:JOIN,list,glue\> <GenEx LIST-JOIN>`, but the two have
   different behavior with regard to empty items.
   :ref:`$\<LIST:JOIN,list,glue\> <GenEx LIST-JOIN>` preserves all empty items,
-  whereas ``$<JOIN,list,glue>`` drops all empty items from the list.
+  whereas ``$<JOIN:list,glue>`` drops all empty items from the list.
 
 .. genex:: $<REMOVE_DUPLICATES:list>